This invention relates to a method and apparatus for determining properties of core samples and, more particularly, to such a method and apparatus where various properties of a geological core sample are determined using pressure transient techniques and variable volume reservoir vessels. Another name for pressure transients is "pressure decay". These two terms will be used interchangeably throughout the description of the invention.
The properties measured from a subterranean rock core sample are important in evaluating the extent and production potential of petroleum and other valuable mineral deposits which may be present in a geologic formation. Property determination of samples from formations currently under production, as well as from formations not yet under production, are of importance.
The detection of heterogeneities within rock samples, the location of fluid fronts or damaged zones, determination of permeabilities of different zones using a single test, and measurement of core porosity or pore volume compressibility are variables of importance in geologic evaluation. In recent years, the exploration and production of petroleum from formations of complex structure has greatly increased. Consequently, the need to measure the properties of many core samples has also accelerated.
While various means of testing core samples to determine bulk permeability exist, the determination of varying properties has been generally through estimation techniques, for example the use of time intensive tracer tests. Bulk permeability is a permeability measurement that assumes the core is homogeneous, and ignores heterogeneities. Permeability measurement using pressure transient techniques, assuming homogeneous cores, was originated by W. F. Brace et al in "Permeability of Granite Under High Pressure," Journal of Geophysical Research (1968), Vol. 73, p. 2225-2236. The technique of Brace has been used by those interested in radioactive waste storage, tight gas sand reservoirs, heavy oil recovery, and low permeability oil field rocks, such as shales. However, the technique of Brace neglected the compressive storage of the rock sample in the development of the analytical equations describing the measurement process. The error in the determination of permeability due to this assumption was minimized by the use of large reservoir vessels in the test apparatus, such that their compressive storage far exceeded the storage of the rock sample.
Others have developed measurement techniques for permeability of core samples, but all prior theoretical analyses assume a homogeneous structure. As stated earlier, physical methods which are capable of quantitatively discerning heterogeneities, such as tracers, are complicated and expensive flow tests to conduct. As a result of such short-comings of prior methods, the method and apparatus disclosed herein were developed. In accordance with the present invention, measurement of multiple properties of a core sample may be made using a single test apparatus and with greater rapidity than previously possible.